1) Field of the Invention
The present invention relates to an image forming apparatus, an optical scanning unit included in the image forming apparatus and a method of correcting positional misalignment adopted in the optical scanning unit.
2) Description of the Related Art
Conventionally, an optical scanning optical system causes scanning velocity unevenness on a surface to be scanned (photosensitive element) due mainly to three reasons described below. As a result, positional misalignment of main scanning dots occurs. In particular, when a color image is formed, the scanning velocity unevenness appears as color drift to cause deterioration in reproducibility of colors and deterioration in resolution.
As a first reason, an fθ characteristic of a scanning lens is not corrected sufficiently.
As a second reason, accuracy of optical components of an optical scanning optical system and accuracy of attachment of the optical components on a housing deteriorate.
As a third embodiment, optical components are deformed due to an environmental fluctuation in temperature, humidity, and the like in an image forming apparatus, and an fθ characteristic deteriorates because a focal length changes due to a fluctuation in a refractive index.
In particular, it is difficult to avoid positional misalignment of main scanning dots due to an environmental fluctuation, which is the third reason, even if optical adjustment or electrical correction is carried out when products are shipped.
Consequently, recently, aiming at improvement in a scanning characteristic, a special surface represented by an aspheric surface has been adopted in an optical element of an optical scanning unit. An “optical element made of resin”, from which this special surface can be formed easily and inexpensively, has been often used. In particular, in an image forming apparatus of a tandem type, since a large number of optical elements are used, the use of the “optical element made of resin” results in significant reduction in cost.
However, since the “optical element made of resin” has a large thermal expansion coefficient compared with glass, a shape of the optical element is deformed largely due to a change in temperature, and an optical characteristic of the optical element changes. Therefore, when temperature in an optical box rises due to deflecting means like a polygon mirror that generates a large amount of heat, the heat is never conducted uniformly due to an air current caused by rotation of the polygon mirror or a difference in a shape in the optical box. As a result, a temperature distribution occurs in the optical box. In addition, in a scanning lens, a uniform temperature change is never caused due to a difference in a way of conduction of heat, a difference in a lens shape (difference in an installation area in the optical box), and the like. As a result, a temperature difference is caused depending on a location of the scanning lens.
Note that, in the image forming apparatus of the tandem type, since light beams directed to respective photosensitive elements pass through different scanning lenses, different temperature distributions occur among the respective scanning lenses depending on a temperature distribution in the optical box holding the scanning lenses. As a result, a change in a shape and a change in a refractive index of the scanning lenses are not uniform, and amounts of a change in a scanning length and changes in uniformity of velocity are different among the respective photosensitive elements. When latent images on these photosensitive elements are visualized by developing devices using developers of different colors like yellow (Y), magenta (M), cyan (C), and black (B) and then the visualized images are sequentially transferred to be superimposed one on top of another and fixed on identical recording paper to form a color image, so-called “color drift” occurs. In particular, when the “optical element made of resin” is used as a scanning lens closest to the deflecting means such as the polygon mirror, which generates a large amount of heat, in the optical box, a change in an optical characteristic of the optical element increases.
Moreover, when images are outputted continuously, in particular, when the number of sheets to be outputted continuously is large, temperature in the image forming apparatus (temperature in the optical box) rises due to heat generation of the deflecting means. Therefore, color drift is caused by changes in temperature distributions of the respective scanning lenses, and an amount of a fluctuation thereof also changes at any time. As a result, a color tint varies in an image outputted first and an image outputted last due to the color drift.
Note that, as a technical document that was filed earlier than the present invention, there is an optical scanning unit that has a writing start position detection sensor disposed in an arbitrary position outside an image area of a photosensitive drum and corrects writing start positions of image formation in respective colors within an error of one clock (see, for example, Japanese Patent Application Laid-Open No. 2000-238319).
In addition, there is an image forming apparatus that adjusts a writing start position and a writing finish position in a main scanning direction to reduce positional misalignment in image formation in the main scanning direction (see, for example, Japanese Patent Application Laid-Open No. 2000-289251).
Further, there is a pixel clock generating apparatus that has a function of shifting phases of signals of an image clock for performing image formation based on phase data according to timing of a clock signal from high-frequency clock generating means. In the pixel clock generating apparatus, data for performing phase shift is set by a unit of data area constituted by plural continuous clock signals (see, for example, Japanese Patent Application Laid-Open No. 2003-103830).
However, in the technologies disclosed in Japanese Patent Application Laid-Open No. 2000-238319 and Japanese Patent Application Laid-Open No. 2000-289251, an influence of positional misalignment of main-scanning dots, which is caused by an optical system or a deflector, cannot be corrected. In addition, in the technology disclosed in Japanese Patent Application Laid-Open No. 2003-103830, phase-shift data for performing phase shift is set by a unit of data area constituted by plural continuous clock signals. This makes it possible to perform control that is less affected by a delay or the like in a data transfer speed when a fluctuation in a phase-shift amount due to changes in various conditions like temperature and elapsed time. However, it is necessary to further reduce color drift to meet a demand for high image quality in recent years.